Series resonant inverters of this kind can be used in an X-ray generator for generating the high voltage as is known inter alia from DE-OS 30 46 413. However, they can also be used for driving the rotor of a rotary-anode X-ray tube or for generating the filament current of the X-ray tube. The thyristors are then conductive during each semi-oscillation of the current through the series resonant circuit. The electric power that can be generated by such an inverter is higher as the semi-oscillations succeed one another more closely.
The semi-oscillations cannot flow directly successively through the push-pull controlled thyristors, as appears from FIG. 1 which represents the variation in time of the current through the series resonant circuit. It is assumed that during the first (positive) semi-oscillation a thyristor is conducting. Subsequent to the zero crossing of the current, the diode connected inverse-parallel to the thyristor first takes over the reverse current and the ignited thyristor is no longer conducting. Due to the power delivered to the user, the negative semi-oscillation of the reverse current (denoted partly by a broken line in FIG. 1) has an amplitude which is lower than that of the positive semi-oscillation.
The thyristor which is active during the next semi-oscillation may be ignited only after the current through the previously conducting thyristor has crossed zero and when subsequently the so-called recovery time (t.sub.q) required for decomposition of the charge carriers has elapsed.
For the recognition of the zero crossing use is made of a zero-crossing detector, usually a Schmitt trigger, which generates a zero-crossing signal as soon as the reverse current has dropped below a value i.sub.s. This value i.sub.s must be high enough so as to prevent an interference signal superposed on the measurement signal from initiating premature generation of ignition pulses. In unfavorable circumstances, however, the load-dependent reverse current is smaller than the value i.sub.s, so that no zero-crossing signal is generated. Consequently, no ignition pulse is generated for the next thyristor so that, when the series resonant inverter is used for generating the high voltage for an X-ray source, an X-ray exposure is prematurely interrupted and the patient being examined is exposed to an unnecessarily high radiation dose because the X-ray exposure must be repeated.